State of the art controllers for fuel burners such as furnaces are now based on microprocessors which dramatically improve the control process. Nevertheless, it is still necessary to provide information as to the current operating state of the fuel burner. Among the most important of the state parameters is whether there is flame in the burner. The continued supply of fuel to the burner must be conditioned on the presence of flame, since if flame is not present and fuel is allowed to flow to the burner, the accumulation resulting can explode or asphyxiate, either one a potentially lethal event. Accordingly, it has been recognized for a long time in burner control technology that detection of flame is of paramount importance.
There are basically three kinds of flame detector elements. Perhaps the most common is the so-called flame rod, which forms with the burner metal a sort of diode element when flame is present arising from the difference in the size of the flame rod compared to the burner itself. An AC potential applied between the flame rod and the burner metal causes DC current to be carried by the ionized particles generated by presence of a flame. By detecting presence of this DC current flow, it is possible to determine presence of flame. Because of the difference in sizes of the flame rod and the burner, the current flow is from the flame rod to the burner, meaning that presence of flame is signified by current flow into the flame rod signal conductor, placing its potential below ground voltage as represented by the burner.
A second type of flame detector is sensitive to infrared radiation, and produces a signal indicating flame when such radiation is present. A third type, and the one with which the invention to be described deals, produces an output when ultraviolet radiation produced by a flame impinges on an ultraviolet detector tube whose impedance drops suddenly in response to the radiation. Each of these sensors produces an output requiring substantial processing by special circuitry before a signal indicating presence and absence of flame and which is suitable to be an input to a microprocessor is generated. The circuitry which converts the flame detector signal to a signal suitable for use by the controller is referred to as a flame amplifier and its output as a flame present signal, or more simply, a flame signal.
The flame amplifier for a UV tube must assure that the impedance change in the UV tube arises from presence of ultraviolet radiation impinging on the tube and not from a high resistance shunt across the tube terminals. An early circuit which discriminates between the sudden change of tube impedance arising from ultraviolet radiation and other types of impedance change between the tube terminals is described in U.S. Pat. No. 4,328,527 (Landis) and having a common assignee with this application.
A flame rod amplifier circuit designed to operate with a positive DC power supply adds a measure of reliability to its operation by interfacing with a flame rod sensor whose output is a negative current, i.e., one whose current flows into the sensor from the flame amplifier. The extra measure of reliability arises from the fact that any leakage current within the flame amplifier cannot masquerade as the negative current flow forming the flame rod output. Any leakage current in a flame amplifier powered by positive voltage will almost invariably be positive, and thus not likely to be interpreted as the negative flame rod sensor output. A pending US patent application which covers a flame amplifier circuit embodying these concepts is titled Fail-Safe Condition Sensing Circuit, has as an inventor Paul Sigafus, was filed on Sep. 30, 1991 with Ser. No. 07/783,950, and has a common assignee with this application.
The most efficient way to implement this flame rod amplifier is as a special purpose microcircuit. Because of this implementation, returns to scale are particularly high, meaning that the unit cost drops substantially with increases in the number of individual circuits produced. Accordingly, it is very advantageous for this flame rod amplifier to be compatible with not only the flame rod detector, but also with the UV and IR detectors. However, the power required to drive the UV and IR detectors is different from that required for flame rod detectors. Accordingly, it is not possible to simply replace the flame rod detector with a UV tube flame detector.
One embodiment of the invention to be described is its ability in one embodiment to interface the above-described flame rod amplifier to the standard UV flame detector tube. This interface circuit provides a flame detector signal when flame is present or absent based on presence of absence of UV radiation and which signal is nearly identical to the signal provided by the flame rod detector in similar circumstances.